The present invention relates to methods of producing hybrid seed in commercial quantities and more particularly to methods using a male-sterile gene.
Hybrid varieties are superior to common open-pollinated varieties. However, hybrid seeds are much more expensive than open-pollinated seeds. For example, hybrid watermelon seed costs ten (for diploids) to one hundred (for triploids) times the cost of open-pollinated seed. One reason hybrid seed is so much more expensive than open-pollinated varieties is because commercial production of hybrid seed requires hand pollination of the flowers of the parent plants that produce the hybrid seed. The seed produced using hand-pollination techniques is estimated to be about fifteen times more expensive than seed produced using open-pollinated methods that rely on natural cross-pollination. Hand-pollination techniques require human visitation to the same female flower on separate days, the concurrent careful protection of female flowers and pollen and/or male flowers, a high degree of skill in thorough pollination of each part of the stigma, and a second covering and marking of the female flower. Considerable skill is required to recognize female flowers that are "ready" for pollination. Moreover, these procedures must be performed at specific times of the day, ideally covering flower buds in the late afternoon and pollinating flowers in the early morning.
One alternative technique demonstrated for watermelon by Mohr et al, Proc. Amer. Soc. Hort. Sci. 65:399-404 (1955), which is hereby incorporated herein by this reference, involves using an incompletely dominant genetic marker in the seed parent line that is interplanted with the pollen parent line and relying on natural cross-pollination to effect hybridization. The recessive genetic marker must express itself in the seedling stage so that it permits separation of hybrid plants from the inbred plants that result from self-pollination. The cost-effectiveness of this technique depends on achieving a high percentage of cross-pollination in order to avoid heavy rates of seeding and of thinning non-hybrid plants from the field. Watermelon is monoecious and thus a good candidate for achieving high rates of natural cross-pollination by interplanting the parent lines. However, the Mohr et al method produced a maximum hybrid percentage yield of only 36%.
The 36% yield of the Mohr et al method is unacceptable for commercial hybrid seed, and brings to mind another reason for the high cost of producing hybrid seed in commercial quantities. That reason is the high cost of testing the lots of hybrid seed to determine the percentage of seed that is the desired hybrid variety. Before selling the hybrid seed to the farmer or grower, the hybrid seed producer must conduct testing of each lot of hybrid seed to determine what percentage of the seed will grow out as the hybrid variety. In conducting this purity testing, about 200 plants will need to be allowed to grow out for two to three months for each lot of the hybrid seed. For watermelon, this planting will require about 25 square feet of isolated field for each of the 200 plants. Moreover, during this two to three month testing period, the seed producer will be unable to sell the hybrid seed undergoing testing, thus delaying recoupment of the investment required to produce the hybrid seed being tested.
Other ways of potentially increasing the percentage of cross-pollination suggested by Mohr et al include: (1) rapid removal of easily accessible staminate flowers from the seed parent line without attempting the time-consuming process of completely eliminating them; (2) use of certain chemical growth regulators that are effective in reducing the ratio of staminate to pistillate flowers in other similar species of plants; (3) optimizing the plant spacing and arrangement for maximum cross-pollination; (4) increasing the population of pollinating insects such as honeybees.
Moreover, the Mohr et al method produced inbreds that were as vigorous as the hybrids and could not be recognized at the cotyledon stage. This raises another concern for the producers and users of hybrid seed in commercial quantities. This concern relates to eliminating off-types in the parent plants used to produce the hybrid variety. To determine whether the parents are pure inbreds, it is necessary to grow out the plants to full maturity, by which time there already may have been cross-pollination with off-types. Because of the presence of off-types, harvesting must be performed selectively from plants that are not off-types. Such selective harvesting is very time-consuming, labor-intensive and expensive.
Besides the Mohr et al method, another alternative technique for watermelon was reported by Watts Proc. Amer. Soc. Hort. Sci. 81:498-505 (1962), which is hereby incorporated herein by this reference, and suggested that a male-sterile character with a distinctive seedling marker would simplify the problem of hybrid seed production. Watts Proc. Amer. Soc. Hort. Sci. 91:579-583 (1967), which is hereby incorporated herein by this reference, concluded that a single recessive gene (ms.sub.g) controlled the male-sterile and glabrous characters in the gamma-ray-induced mutant from the "Sugar Baby" variety of watermelon. However, commercial seed production was not feasible because the gene caused female-sterility (partial to severe) in the diploid state and almost complete female-sterility in the tetraploid state. Moreover, fully acceptable melon quality had not been coupled with strong resistance to fusarium wilt and resistance to anthracnose.